Many Active Queue Management (AQM) schemes have been proposed in the last 20 years since Random Early Detection (RED) was first presented in S. Floyd and V. Jacobson, “Random early detection gateways for congestion avoidance,” IEEE/ACM Transactions on Networking, 1(4):397-413, 1993. As opposed to the conventional tail-drop policy, which drops an incoming packet only if the packet finds the queue fully loaded, AQM schemes start dropping packets when the queue is still far from filling up. By proper spacing of the early packet drop decisions, AQM balances the amount of packets that are removed from the data path because of the reduced activity of the TCP sources that experience the packet losses with the amount of packets that all the other sources keep adding to the data path.
The performance of AQM schemes has been studied extensively, but almost exclusively under steady bottleneck rates. Since standard frameworks for Quality of Service, see for example, S. Blake et al., “An architecture for differentiated services,” IETF RFC 2475, December 1998, and IEEE 802.1Q <http://www.ieee802.org/1/pages/802.1Q-2011.html>, typically assign lower scheduling priority to TCP versus other types of traffic, in practice the output rate of the TCP buffer is continuously modulated by the arrival rate of higher-priority traffic. The rather negative effects of bottleneck rate variations on the link utilization performance of buffer management schemes are documented in Y. Zheng, M. Lu, and Z. Feng, “Performance evaluation of adaptive AQM algorithms in a variable bandwidth network,” IEICE Transactions on Communications E86-B(6):2060-2067, June 2003 and J. Zhou, F. Ren, and C. Lin, “Modeling the effects of variable bandwidth on TCP throughput,” ICCCN 2009, San Francisco, Calif., August 2009. In addition, the results presented in A. Baiocchi and F. Vacirca, “TCP fluid modeling with a variable capacity bottleneck link,” IEEE INFOCOM 2007, Anchorage, Ak., May 2007, indicate that those effects are mostly controlled by the relationship between the round-trip time (RTT) of the TCP connections and the fundamental time constant of the bottleneck rate function (BRF), which represents the evolution of the bottleneck rate over time. There is very little that an AQM scheme can do to prevent the TCP buffer from overflowing when the amplitude of the bottleneck rate variations is large compared to the size of the buffer. However, an effective AQM scheme can help prepare the TCP connections for a faster throughput recovery, so that the long-term consequences of the buffer overflow events are much milder.
What is desired are methods for improving the response of AQM schemes to bottleneck rate variations, so that the negative impact of those variations on TCP throughput is minimized.